Chris, I don't completely understand if you want to know about chmod or specifically 644, so I'll provide a little about both.

First, to change permissions for a file (examle filename.cgi), you need to chmod the file (change mode). For example, when you type this:

chmod u=rwx,g=rx,o=rx filename.cgi

you've given:
read, execute, and write access to the user (that's you)
read and execute access to the group and
read and execute access to others

Some scripts will tell you to chmod 775 (for example). Doing the above is the same thing as typing chmod 775.
You can use either method with our Unix servers. Let me explain:

When using the numeric system, the code for permissions is as follows:

r = 4 w = 2 x = 1 rwx = 7

The first 7 of our chmod775 tells Unix to change the user's permissions to rxw (because r=4 + w=2 + x=1 adds up to 7. The second 7 applies to the group, and the last number 5, refers to others (4+1=5).

I. Introduction
There's an interesting discussion going on now in an Experts Exchange Group — Attachments with no extension (http://www.experts-exchange.com/discussions/210281/Attachments-with-no-extension.html). This reminded me of questions tha…